Variable displacement type hydraulic motors have been hitherto practically used as a driving power source for running a construction machine such as a power shovel or the like. Each hydraulic motor of the foregoing kind is constructed such that its rotational speed can variably be changed by way of two steps to assume either of a high speed (low torque) and a low speed (high torque) so as to properly cope with variation of the running state of the construction machine. In some case, a speed changing operation to be performed by way of two steps is manually accomplished. For example, when the construction machine runs on a flat road surface, a swash plate angle of each hydraulic motor is shifted to the position corresponding to a high speed (low torque) by operator's actuation. In addition, in a case where the construction machine is required to run at a low speed or when a high torque is required so as to allow the construction machine to run on the road surface having a certain gradient, the swash plate angle is shifted to the position corresponding to a low speed (high torque) by operator's actuation so as to allow the construction machine to run while changing its running speed by way of two steps. According to the prior inventions disclosed in official gazettes of Japanese Utility Model Application NO. 61-148732 and Japanese Patent Application NO. 62-19768, a swash plate variable cylinder corresponding to a high speed and a swash plate variable cylinder corresponding to a low speed are arranged separately for a high rotational speed and a low rotational speed of each hydraulic motor so as to vary a swash plate angle of each hydraulic motor, and moreover, a solenoid valve is arranged for selectively supplying a pressurized oil to either of the swash plate variable cylinder for a high speed and the swash plate variable cylinder for a low speed so as to properly perform an automatic speed changing operation in conformity with the present running state of a vehicle by detecting the present pressure of the pressurized oil discharged from a hydraulic pump rotationally driven by an engine and then properly controlling the solenoid valves based on the result derived from the pressure detection.
With respect to the conventional automatic speed changing technology as mentioned above, it has been found that there arise the following malfunctions while a vehicle runs in a transitional state.
Specifically, while a swash plate angle of each hydraulic motor is set to the position corresponding to a low speed before a vehicle such as a construction machine or the like starts to run, there does not appear any particular problem. On the contrary, in a case where a swash plate angle of each hydraulic motor is set to the position corresponding to a high speed, there arises a malfunction that a speed changing operation can not be correctly controlled.
For example, in a case where a vehicle runs at a very slow speed while an engine is driven at a low rotational speed or in a case where the vehicle starts to run with a small quantity of actuation of each actuation lever for running the vehicle, i.e., at a reduced flow rate of a pressurized oil for driving the vehicle, there is no need of running the vehicle at a high speed. However, in a case where the vehicle runs on the ground surface having a certain gradient or turns on the road surface while performing a controlling operation for changing the present speed to a high speed or a low speed as a load pressure exerted on each hydraulic motor is detected on the assumption that a swash plate angle is set to the position corresponding to a low speed, there arises another malfunction that the vehicle undesirably runs in a hunting state.
In addition, in a case where the vehicle runs on the ground surface having a certain gradient while a swash plate angle is set to the position corresponding to a high speed, when an actuating lever starts to be actuated, no torque is generated with each hydraulic motor having its swash plate angle set to the position corresponding to a high speed. Subsequently, as an actuation lever is actuated, a hydraulic pressure is raised up, causing the swash plate angle to be shifted to the position corresponding to a low speed. As a result, a certain torque is rapidly generated with each hydraulic motor. Thus, the vehicle is brought in such a very dangerous state that the vehicle starts to run abruptly.
The present invention has been made with the foregoing background in mind and its object resides in providing an apparatus for variably controlling a running speed of each hydraulic motor wherein the present running state of a vehicle is determined in consideration not only of a running load of each hydraulic motor but also of other factors, the rotational speed of each hydraulic motor set by an operator is corrected based on the results derived from a series of determinations, and moreover, the running speed of each hydraulic motor is variably controlled so as to obtain the corrected rotational speed, whereby the vehicle can smoothly run at all times.